Can I work on a page as a draft and undo changes if I make a mistake?

Yes to both, with one caveat.

Draft: on the page's edit screen, set Page status to Draft (other options: Publish and Trash) and click Save changes. Draft pages are left out of your store's page list and sitemap. They can still open, though, if someone has the exact address. So don't link a draft from your menu until it's ready.

Undo: click Version history at the top of the edit screen. WCart saves a copy automatically before each change and keeps the latest 20 versions. Click Restore next to the version you want. Your current content is saved as a new version first, so restoring is safe. Blog posts have the same Version history button.